Zinc is an essential micronutrient for all living organisms including plants. Zinc is typically the second most abundant transition metal in organisms after iron (Fe), and the only metal represented in all six enzyme classes (oxidoreductases, transferases, hydrolases, lyases, isomerases and ligases). Zinc binding sites also occur in a wide range of other proteins, membrane lipids and DNA/RNA molecules. The largest class of Zn-binding proteins in organisms is the zinc finger domain containing proteins that function as transcription regulator.
Zn is present in the soil in primarily three fractions: (i) water-soluble Zn (including Zn2+ and soluble organic fractions), (ii) adsorbed and exchangeable Zn in the colloidal fraction (associated with clay particles) and (iii) insoluble Zn complexes and minerals. Zinc is acquired from the soil solution primarily as Zn2+, but also potentially complexed with organic ligands, by roots which feed their shoots via the xylem.
When facing a shortage in zinc supply, plants adapt by enhancing the zinc uptake capacity. Plants are thought to control Zn homeostasis using a tightly regulated network of zinc status sensors and signal transducers controlling the coordinated expression of Zn transporters involved in Zn acquisition from the soil, mobilization between organs and tissues and sequestration within cellular components (Clemens, S., 2001, Planta 212:475-486).
In recent years numerous studies have been performed to unravel the biochemical pathways of Zn uptake and transport. In these studies, however, it is not yet found which proteins are responsible for Zn uptake from the soil (Palmer, C. M. and Guerinot, M. L., 2009, Nature Chem. Biol. 5:333-340). While candidate genes for the required Zn transporters have been identified, the so-called ZIP transporters ZIP1, ZIP2, ZIP3 and ZIP4 (Grotz, N. et al., 1998, Proc. Natl. Acad. Sci USA 95:7220-7224), more proteins seem to be necessary to explain the phenomenon of Zn-hyperaccumulation, such as the heavy metal transporters HMA2, 3 and 4 (Hanikenne et al, 2008, Nature 453:391-395). Also nicotianamine, made by nicotianamine synthase (NAS), seems to play a role in vascular transport of zinc, and the citrate transporter FRD3 (Durrett, T. P. et al., 2007, Plant Physiol. 144:197-205). For transport between tissues and organs several proteins seem to be involved. Members of the YSL group, a subfamily of the oligopeptide transporter (OPT) family of trasnporters, are proteins that have been mentioned. For intracellular transport of Zn, NRAMP ZIP and ZIF proteins seem to be involved. Transport of Zn into the vacuole is performed by MTPs (metal tolerance proteins, also referred to as cation diffuser facilitator—CDF—proteins).
(Lack of) adaptation of plants to a changing concentration of Zn in the environment works both ways: zinc deficiency and zinc toxicity. After Fe deficiency, Zn deficiency is the most commonly occurring micronutrient deficiency in agriculture, mainly affecting parts of Asia (Turkey and Near-East Asia, Central Asia, South and Central China), Sahel and sub-Saharan Africa and Australia. Since plants are often the major dietary source of micronutrients for human consumption, many people worldwide suffer from Fe and Zn deficiencies as plants, especially cereals, are notoriously poor in their content of bioavailable Fe and Zn.
Some soils are not deficient in minerals, but have become contaminated with large amounts of heavy metals, such as Zn or Cd. Zn toxicity in crops is far less widespread than Zn deficiency. However, toxicity symptoms usually become visible at Zn concentrations of more than 300 mg Zn kg−1 in the leaves. Some plants are known to be able to grow on soil that has a high concentration of Zn, such as Silene vulgaris, Thlaspi caerulescens, Arabidopsis halleri and Viola calaminaria (see also Table 3 in Broadley, M. R. et al., 2007, New Phytologist 173:677-702). It has been suggested to use these Zn hyperaccumulators as sanitation plants to extract zinc from the soil, whereby the metal is concentrated in the biomass, that can be harvested, incinerated and properly disposed of. Such a method, known as phytoremediation, is currently not attractive as the known metal hyperaccumulator plants are relatively small, thus producing not sufficient biomass to yield a high metal extraction capacity.
Thus, there is still need to be able to control the adaptation of plants to changes in zinc concentration in the environment.